Standard atmospheres to Torr (atm to Torr)

One standard atmosphere (atm) is defined as exactly 101,325 pascals (101.325 kPa) by the 10th CGPM resolution of 1954. This value is also equal to exactly 760 millimetres of mercury (mmHg, or torr) at 0 °C under standard gravity — the equivalence is by definition, not by measurement, and was specifically constructed so that the older mercury-column convention and the pascal-based SI convention give the same numerical reference. Conversions to other commonly-encountered pressure units: 1 atm = 1.01325 bar exactly, 1 atm = 14.6959488 psi, 1 atm = 29.9213 inches of mercury, and 1 atm = 1,013.25 millibar / hectopascal. The unit symbol "atm" is recognised by the BIPM but explicitly listed as a non-SI unit whose use is discouraged in favour of the pascal — yet it persists in chemistry, hyperbaric medicine and diving because the magnitude is human-scale and the gas-law formulas are taught in terms of it. A closely related notation, "ATA" (atmospheres absolute), is used in diving and hyperbaric work to make explicit that the pressure is referenced to a perfect vacuum rather than to local atmospheric pressure — so 1 ATA at the surface, 2 ATA at 10 metres of seawater, and so on.

The standard atmosphere descends from Evangelista Torricelli's 1644 barometric experiment in Florence, in which a glass tube sealed at one end and filled with mercury was inverted into a dish of mercury; the column settled at about 760 mm above the dish, balanced by atmospheric pressure on the dish surface. Pascal's 1648 Puy de Dôme expedition extended Torricelli's work to confirm altitude dependence, and the 760 mm value became the conventional reference for "one atmosphere" of pressure across the eighteenth and nineteenth centuries. Through that period the standard atmosphere consolidated as the reference pressure for gas-law work — Boyle's law (1662), Charles's law (1787), Avogadro's hypothesis (1811), and the unified ideal-gas equation pV = nRT all referenced atmospheric pressure as the natural baseline, and chemistry developed a deep practical reliance on the atm as the working pressure unit for laboratory calculations. The modern numerical definition was fixed by the 10th General Conference on Weights and Measures (CGPM) in 1954 — the same conference that defined the kelvin against the triple point of water — at 1 atm = 101,325 Pa exactly. The CGPM framed the value as the mean atmospheric pressure at sea level, 45° latitude, 0 °C: an idealised reference rather than a directly measured quantity, closing nearly two centuries of small national variations in the assumed value. In 1982, IUPAC redefined "standard temperature and pressure" (STP) from 0 °C + 1 atm to 0 °C + 1 bar (100,000 Pa) for SI alignment. The older 1 atm definition remains in the educational literature and many engineering reference works, and the resulting "old STP" / "new STP" split is the most persistent legacy issue in introductory chemistry pedagogy.

Chemistry and gas-law calculations are the centerpiece of atm's active educational and laboratory use. Every introductory and physical-chemistry textbook in the English-speaking world (Atkins, McQuarrie-Simon, Levine, Zumdahl) presents the ideal-gas equation pV = nRT with worked examples in which the pressure variable carries units of atm, and the universal gas constant R is most commonly memorised in its convenient form 0.08206 L·atm·mol⁻¹·K⁻¹ rather than its SI form 8.314 J·mol⁻¹·K⁻¹. Dalton's law of partial pressures, Henry's law for gas solubility, Raoult's law for vapour pressure of solutions, and the equilibrium constant Kp (defined in terms of partial pressures) all conventionally use atm as the reference pressure. The molar volume of an ideal gas at the older STP convention (0 °C, 1 atm) is the famous 22.414 litres per mole, a value memorised by chemistry students for almost a century — superseded numerically but not pedagogically by the 22.711 L/mol of the post-1982 STP convention. Diving and decompression theory: recreational and technical diving teaches depth-pressure as multiples of one atmosphere absolute (ATA), with one additional atmosphere added for every ten metres of seawater depth. A diver at 10 m experiences 2 ATA, at 20 m experiences 3 ATA, at 30 m (the recreational limit on air without decompression-stop training) experiences 4 ATA, and a technical diver at 60 m experiences 7 ATA. The US Navy Diving Manual decompression tables, the PADI and SSI recreational dive tables, and the algorithm-driven dive computers (Bühlmann ZH-L16, VPM-B, RGBM) all denominate ambient pressure in ATA for the decompression-modelling calculations even when the cylinder gauge on the same dive reads in bar. Henry's law gas-loading and off-loading from body tissues — the foundation of decompression theory — is computed in terms of ambient ATA partial pressures of nitrogen and helium. Hyperbaric medicine: clinical hyperbaric oxygen therapy (HBOT) prescribes treatment pressures explicitly in ATA. The Undersea and Hyperbaric Medical Society (UHMS) approved indications for HBOT specify treatment regimes typically at 2.0–2.4 ATA for chronic wound healing, diabetic foot ulcers, radiation tissue injury, and carbon monoxide poisoning, with severe decompression sickness and arterial gas embolism treated under US Navy Treatment Table 6 at 2.8 ATA. Hospital monoplace and multiplace hyperbaric chambers display chamber pressure in ATA on the operator console as the primary clinical variable. Autoclave sterilisation: hospital and laboratory steam autoclaves operate at approximately 1 atm gauge pressure (about 2 ATA absolute) at 121 °C for 15–30 minutes per the CDC's Guideline for Disinfection and Sterilisation in Healthcare Facilities and the ANSI/AAMI ST79 comprehensive sterilisation guide; the higher 134 °C "flash" cycle uses about 2 atm gauge (3 ATA absolute) for shorter exposure. ASME Boiler and Pressure Vessel Code Section VIII pressure-vessel calculations for autoclave design carry the ratings in psi for the US market and bar for European, but clinical and microbiological literature consistently report cycle parameters in atm. Planetary science uses atm as a convenient ratio reference for comparing other planetary atmospheres to Earth's: the surface pressure of Venus is about 92 atm (a crushing nine-kilometre-deep ocean equivalent in pressure terms), Mars surface is about 0.006 atm (well below the Armstrong limit of 0.0618 atm at which water boils at body temperature), Titan's surface is about 1.45 atm, and the gas-giant atmospheres are conventionally measured against the 1 atm "surface" level of their pressure profiles since they have no solid surface.

One Torr is defined as exactly 1/760 of a standard atmosphere, which equals exactly 101,325/760 = 133.322368... pascals. The relationship to the millimetre of mercury (mmHg) is very close but technically distinct: 1 mmHg = 133.322387415 Pa per the BIPM definition based on standard gravity acting on a column of mercury, while 1 Torr = 133.322368... Pa per the 1/760-atmosphere definition. The two values differ by about 0.000015% and are interchangeable for all practical purposes outside high-precision metrology. The recognised symbol is "Torr" (capitalised, in honour of Torricelli), occasionally seen as "torr" in casual writing but BIPM convention preserves the capital. Vacuum-technology pressure ranges span from ultra-high vacuum (10⁻⁹ Torr and below) through high vacuum (10⁻³ to 10⁻⁹ Torr), medium vacuum (10⁻³ to 1 Torr), low vacuum (1 to 760 Torr) up to atmospheric (760 Torr). Above atmospheric the unit is rarely used; pressure scales transition to bar or kPa.

The Torr is named after Evangelista Torricelli (1608-1647), the Italian physicist and mathematician who in 1643 demonstrated atmospheric pressure with the first mercury barometer — a sealed glass tube inverted in a dish of mercury, with the mercury column standing at the height proportional to atmospheric pressure. Torricelli's barometer experiment, originally proposed by Galileo and executed by Torricelli in his role as Galileo's secretary, established that air has weight and that a column of mercury about 760 mm tall at sea level is equal in weight to the column of air above it. The unit named in Torricelli's honour was formally adopted at the BIPM in 1958 as exactly 1/760 of a standard atmosphere, making it numerically very close to but legally distinct from the millimetre of mercury (mmHg). Both units are used in vacuum technology, with Torr being the dominant convention in physics laboratories and high-vacuum technology and mmHg dominating in clinical medicine and meteorology. The Torr is not part of the SI but is recognised by NIST and BIPM as a non-SI unit accepted for limited use; ISO 80000-4 deprecates it in favour of pascals for new technical writing.

Vacuum technology and high-vacuum laboratory work: physics-laboratory vacuum chambers, semiconductor-fabrication chambers, mass-spectrometry source pressures, electron-microscopy vacuum levels and surface-science laboratory work all denominate working pressures in Torr. Ultra-high-vacuum chambers operate at 10⁻⁹ to 10⁻¹² Torr; high-vacuum lithography systems at 10⁻⁶ to 10⁻⁸ Torr; mass-spectrometer source regions at 10⁻⁵ to 10⁻⁷ Torr; electron-microscope columns at 10⁻⁴ to 10⁻⁶ Torr. Edwards, Pfeiffer, Leybold and Agilent vacuum-equipment manufacturers all preserve Torr alongside Pa and mbar on their product specs and pressure-gauge displays. Semiconductor manufacturing: photolithography, plasma etching, chemical-vapour deposition (CVD) and physical-vapour deposition (PVD) chambers all run at sub-atmospheric pressures denominated in Torr in industry-standard tooling specifications. ASML, Lam Research, Applied Materials and Tokyo Electron all use Torr alongside Pa on their semiconductor process-tool spec sheets. Pressure-swing adsorption gas separation: industrial gas-purification systems (oxygen concentrators, hydrogen purifiers) cycle between low-Torr and atmospheric pressures with Torr as the conventional low-side pressure unit. Medical vacuum: chest-tube drainage and surgical-suction equipment operates at Torr-scale pressures (50-200 mmHg/Torr below atmospheric), with the Torr-vs-mmHg distinction immaterial at the precision required for clinical use.